UniSimDesignTutorialfor3rdyearmodule2008

UniSim Design Tutorial for CHEE 399

Queen’s University Department of Chemical Engineering

2008

Introduction

UniSim (very similar to HYSYS) is a program used to design chemical plants. It is built around:

a library of the physical properties of a large number of chemical species a set of subroutines to estimate the behavior of many types of plant equipment (heat

exchangers, reactors, etc.) a graphical user interface to accept specifications for the case, and display results

The user describes the process in terms of pieces of equipment interconnected by process streams, and the program solves all the mass/energy/equilibrium equations, taking into consideration the specified design parameters for the units.

In this tutorial, you will be introduced to some of the basic features of UniSim. Once you have completed this tutorial, it is highly recommended that you attempt to work through the advanced tutorial (available for download on the course webpage), as it will introduce you to additional unit operations and features. Upon completion of both tutorials, you will be well-prepared to dig into the plant optimization assignments.

Separating CO2 from Synthesis Gas

In this year’s Integrated Design Project, we are interested in developing a process to obtain pure hydrogen (H2) from Synthesis Gas (syngas) which, for the purposes of this assignment, is a mixture of CO2, CO, H2 and H2O. The reaction governing the ratio of these gasses is:

CO + H2O ↔ H2 + CO2

If we drive the reaction equilibrium towards the production of H2, we will also increase ratio of CO2 in the gas mixture. This CO2 is then can be removed from the syngas using Selexol, a liquid absorbent. Since Selexol is a physical absorbent, decreasing the temperature and increasing the pressure will drive CO2 to dissolve into the liquid. Downstream, the CO2 can then be removed by decreasing the pressure which will cause the solvent to degas.

The amount of CO2 that the Selexol is capable of absorbing can be calculated based on the principles of vapour/liquid equilibrium (VLE). Since Selexol and CO2 are not chemically similar, gas solubility cannot be accurately predicted using an unmodified Raoult’s law (which would assume the fluids are ideal). Our choice of property package will determine how the model behaves when operating outside of conditions where ideality holds. If we use the UNIQUAC (UNiversal QUAsiChemical) model, gas solubility can be calculated using a set of

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binary coefficients. Experimental values for these binary coefficients are unavailable in the UniSim database. The UNIFAC (UNiversal Functional Activity Coefficient) method can be used to generate semi-theoretical estimates of these binary coefficients based on molecular size and shape differences and molecular interactions in the system. These binary coefficients can then be validated by comparing the solubilities predicted by the simulation to the manufacturer’s data (refer to Genosorb_data.pdf – use Genosorb 1753). If the solubilities are not being predicted correctly, the binary coefficients can be modified using an educated trial-and-error basis.

We will develop a model capable of performing the solubility calculation. The binary coefficients of the UNIQUAC model will be adjusted to match the manufacturer’s solubility data. These binary coefficients will then be input into the Integrated Design Project process model.

The following diagram shows the process model we will use in our solubility calculation for CO2

in Selexol.

To ensure the Selexol is saturated with CO2, a stream containing excess amounts of CO2 is mixed with a stream of pure Selexol. This mixed stream will contain two phases: a liquid stream of Selexol saturated with CO2, and a vapour stream consisting of excess CO2. The excess CO2 (beyond saturation) is removed using a (drum) separator, leaving us with the CO2-saturated Selexol stream. A component splitter (aka a “black box”) separates the CO2 from the Selexol, allowing us to read off the normal volume of CO2 gas that was dissolved in the Selexol on a per kg basis, just as it is reported in the manufacturer’s data file. The binary coefficients can then be easily adjusted until the model predictions match the experimental data.

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Setting up Data for the Model

Open UniSim (Accessed under Honeywell from the Start menu in Windows).

Click on the New File icon and get:

Thermodynamics

Click on the Fluid Pkgs tab.

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In UniSim, a “Fluid Package” includes all the thermodynamic, component and reaction parameters required to run the model. It is possible to have multiple packages in a model. For example, in a more complex plant model, one could add more than one fluid package if they wanted to use one particular thermodynamics model for the reactors and a different thermodynamics model for other unit operations.

Click on Add and scroll to find UNIQUAC, since we are using the UNIQUAC method to model the thermodynamics of our system.

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Note that default name of the package is “Basis-1” and our component materials will go into “Components List - 1”.

Adding Components

Close the “Fluid Package” window and click on the Components tab of the “Simulation Basis Manager” window.

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Select Component List – 1 and click on View.

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We will now add the four components of the Syngas mixture (CO, H2O, H2, and CO2) to our components list. Let’s begin with CO by entering “Carbon_Monoxide” in the “Match” field. Notice that if you forget the underscore ( _ ), the search will not turn up any components!

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Click on Add Pure and do the same with “Water”.

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Often it is easier to search for and add compounds using their chemical formulas. Select the Formula option and search for “H2”. Add Hydrogen to the “Selected Components” list.

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Do the same thing with “CO2”.

The final component we will add is Selexol. Select the Full Name / Synonym option and search for “Selexol” and add it to the list of components.

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Close the components window. Now that we have finished adding all of the components, it would be a good idea to save the case. As with most Windows programs, this can be accomplished by clicking on File → Save. Save the file as 399tutor01.

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Estimating Binary Coefficients with UNIFAC

The UNIQUAC model requires a set of binary coefficients. These can be estimated using the UNIFAC method. From the “Simulation Basis Manager” window, click on the Fluid Packages tab. Select Basis-1 and click on the View button. Now, click on the Binary Coefficients tab to access the “Activity Model Interaction Parameters” window.

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We are interested in the bottom row, the binary coefficients that mathematically describe the interaction between the component gasses when dissolving in Selexol.

Since the UNIFAC VLE option is automatically selected, click on the Unknowns Only button to obtain estimates for binary coefficients that are were not available in the database.

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Now close the Fluid Packages window. It is now time to exit the Basis Manager and enter the Simulation Environment. To proceed with building the process model, click on the Return to

Simulation Environment” button. You can also click on the button in the main toolbar.

A warning window will pop up. Click Ok.

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Building the Process Flow Diagram

Save your progress. If you make a mistake in your model, particularly one which causes UniSim to crash at an inopportune moment, it is always handy to have an earlier version of the model saved so you can quickly return to a case that worked. If you haven’t developed a habit of saving your work often, now is a good time to do so! Do a “Save” followed by “Save As 399tutor02”.

This is the blank screen on which you will model the process by building a Process Flow Diagram (PFD).

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The PFD operations palette, as shown on the left, should be visible. If it is not, click on Flowsheet →Pallette.

We will begin by defining creating two feed streams into the system. Double

click on the blue material stream icon to bring up an empty stream window.

Note that the properties are being estimated using the Basis-1 package.

We will begin with Selexol feed stream. Enter the following data:

Steam Name = Selexol_In

Temperature = 25 ℃ Pressure = 100 kPa

Mass Flow = 1000 kg / h

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To finish the stream definition, we need to specify the composition. Click on Composition on the left side of the window.

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Click on field next to Selexol and enter “1”. A new window will pop up. Click on the Normalize button.

Now, click OK. Click on Conditions to go back to the “Conditions” window.

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The bar near the bottom will turn green (“OK”) to indicate that the stream is now fully-defined. UniSim has calculated the variables that you did not specify and they show up a black font. From now on, you will only be able to change values that show up in a blue font, not the black ones. Close the feed stream window.

Now we will repeat the process to define the CO2 feed stream. On the operations palette, double

click on the blue material stream icon to bring up an empty stream window. Enter in the following information:

Steam Name = CO2_IN

Temperature = 25 ℃ Pressure = 100 kPa

Mass Flow = 1000 kg / h

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Click on Composition and click on the field next to CO2. Enter in “1”. The composition input window will open up.

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Click Normalize then OK to exit this window. Clicking on Conditions we can see that this feed stream has now been defined.

As expected, CO2 at 25℃ and 100 kPa (1 bar) exists as a vapour, as indicated by a “Vapour / Phase Fraction” equal to 1.0. We are now done specifying the CO2 feed stream, so close the feed stream window.

Now it is time to add a mixer to combine the two feed streams. Double click on the mixer icon

in the operations palette.

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In the “Name” field, specify “Feed Mixer”. Then click on <<Stream>> in the “Inlets” list. You will be presented with a list of acceptable steams. Select “CO2_IN”.

Now we will input the second input stream. In the second <<Stream>> line, select “Selexol_IN”.

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UniSim informs us that we still need to specify a product stream. It often pays to check the status bar at the bottom of the page when you are stuck. Sometimes it can provide helpful hints and remind you if you’ve forgotten to specify an important design parameter.

Next, click on the box below Output and you will see that there are no candidate streams. So enter the name “Mixed Feed”.

The mixer has now been sufficiently specified. We have chosen to accept the default operating parameters. Clicking on Parameters (on the left), will bring up the following screen.

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Each type of operation will have a set of parameters that may be adjusted. The parameters are usually specific to the type of operation you are specifying. For example, pump efficiency is one of the parameters that may be adjusted when you are designing a pump. For (drum) separators, the parameters screen allows you to specify the pressure drops at the both the inlet and vapour outlet (by default they are set to zero).

For our mixer operator, there is only one parameter that can be changed: the method by which the outlet pressure is calculated. There are two options.

When “Equalize All” is selected, UniSim checks that all of the inlet streams have the same pressure. If this is the case, the outlet stream pressure is set to equal the pressure of the inlet streams. If one (or more) of the inlet streams has a different pressure, UniSim will display a warning, informing you that the system is over-specified. This problem can be fixed by going back and changing the pressure specification of the offending inlet stream (making sure that all your inlet streams have the same pressure).

When “Set Outlet to Lowest Inlet” is selected, UniSim sets the pressure of the exit stream to equal the lowest inlet stream pressure. When this option is selected, the inlet streams can have different pressures. In a real plant, the inlet streams to a mixer will have the same pressure, so avoid selecting this option if you can.

By default, the “Equalize All” radio button is selected and we do not want to change this. Close the window and take a look at the main screen.

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All of the streams are dark blue, indicating that UniSim is “happy” (that all the conditions required to ‘fullly’ specify a stream have been made, and that all resulting phsyical properties have been calculated)

The Flash System

Once the feed streams are mixed together, the Selexol will become saturated with CO2 from the feed stream. The “Mixed Feed” stream will consist of two phases: a liquid stream (Selexol saturated with CO2) and a vapour stream (the CO2 left over after the Selexol is saturated). We need a separator to isolate the two phases.

Double click on the separator icon on the operations palette. This will bring up a screen where you can specify the separator design.

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Rename the unit by typing “Separator” into the Name box. Attach the “Mixed Feed” stream by clicking on the box below Inlets and selecting it. In the Vapour Outlet box, type “Excess CO2”. In the Liquid Outlet box, type “Saturated Selexol”.

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Note that according to UniSim, the unit has been sufficiently specified. We could, should we wish to, alter some of the design specifications. If we wanted to specify a pressure drop across the inlet and/or vapour outlet, we could click on Parameters which would bring up the following screen:

The numbers in both boxes are blue, indicating that we could choose to specify pressure drops or specify other parameters. However we will stick with the defaults.

Close the Separator window and do a “Save” followed by a “Save As 399tutor03”. Your PFD should look similar to this one.

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Double click on the “Excess_CO2” stream. This will open up a new window giving us information on the vapour outlet stream.

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Take a look at the mass flow rate. Assuming the stream is essentially pure CO2, it appears that of the 1000 kg of CO2 that we feed in per 1000 kg of Selexol, 2 kg of CO2 will absorb into the Selexol. Click on Composition on the left side of the window.

This confirms that the stream is essentially pure carbon dioxide.

Our model predicts that 1000 kg of Selexol will absorb 2 kg of CO2 at 25℃ and 1 bar of pressure. However the manufacturer’s specifications list gas solubility in terms of N cm3/g ∙ bar. Note that N stands for normal and not Newton and refers to a predefined set of temperature and pressure conditions. Since we’ve chosen to operate at 1 bar (100 kPa), we can simplify the units to N cm3/g, but this information is still unavailable with the model in its present form.

Splitting Components

To determine the normal volume of gas capable of absorbing into a kg (or g) of Selexol, we must split the components of the “Saturated Selexol” stream.

On the operation palette, double click on the component splitter icon.

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Change the name of the unit to “Splitter” and specify the input and output streams as shown.

Click on Splits and click on the box immediately to the right of “CO2”.

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Click on Set to 1.0000 to specify that the “Absorbed CO2” stream will contain CO2 only. Set the remaining components to 0.0 by clicking on each of the corresponding boxes and clicking Set to 0.00000.

Click on the Worksheet tab near the bottom of the window.

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We must specify the split stream pressures of both streams to 100 kPa, as shown.

The component splitter has not been sufficiently specified, so UniSim is unhappy. If we look at the message window near the bottom of the main window, we can gain an clue as to which information is missing.

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UniSim is telling us that the temperature of the “Selexol_OUT” stream is unknown. Specifying the temperature of one of the outlet streams might fix this problem. Since we need to measure

normal CO2 volume at 25 ℃ , set the temperature of the “Absorbed CO2” stream to 25 ℃.

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The component splitter design has now been sufficiently specified. Close the “Splitter” window. Your process stream should now look similar to the one below:

Click on the “Absorbed CO2” stream.

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We already knew the mass flowrate of CO2 was around 2 kg. The Conditions window does not tell us what the normal volumetric flow rate of the CO2 stream. Click on Properties to bring up the properties window.

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It is empty, so we must specify which additional properties we want UniSim to calculate. On the main menu bar, click on Tools → Preferences… to bring up the “Session Preferences” window.

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Check the Activate Property Correlations (Standard, Black Oil, Electrolyte) box.

Click on the Variables tab near the bottom of the window. In the Available Unit Sets box, click on “SI” and click on the Clone button.

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Rename the new unit set as you please (In the tutorial, it has been renamed “CHEE 399”) and make sure it is selected. In the Display Units box, scroll down until you find “Std. Gas Flow”. Change the unit from “STD_m3/h” to “Nm3/h” as shown.

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Now that you have specified the use of a customized set of variables where the Std. Gas Flow is reported in N m3/ h, click on Save Preferences near the bottom of the window to save your preferences.

Close the preferences window and click on the “Absorbed CO2” stream on the PFD to open up the corresponding stream properties window.

Click on Properties and scroll down until you can read off the “Std. Gas Flow” rate.

According to the model, a stream of Selexol flowing at 1000 kg / h is capable of absorbing the equivalent of 0.9953 N m3 / h of CO2. In other words, the solubility of CO2 in Selexol is 0.9953 N cm3 according to our model. This is less than the experimental solubility value supplied by the manufacturer (3.1 N cm3) so we will have to adjust the corresponding binary coefficient until our process model prediction matches the manufacturer’s data.

Adjusting the Binary Coefficients

Without closing the “Absorbed CO2” properties window, click on the “PFD” window and minimize it.

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Now would be a good time to save your work. Save the latest version of your document as “399tutor04”.

Click on the beaker icon to exit the simulation environment and re-enter the basis environment. If it isn’t already selected, click on the Fluid Pkgs tab and click the View… button to edit the fluid package.

Now, click on the Binary Coeffs tab to bring up the binary coefficients window.

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The binary coefficient for CO2 in Selexol that was estimated using the UNIFAC method must be adjusted to ensure that the CO2 solubility predicted by the UniSim model matches the solubility data provided by the manufacturer. The binary coefficient that you will have to adjust has been circled above.

Since we do not know whether we need to increase or decrease the binary coefficient, let’s begin by changing it to 69.000 and pressing Enter (or click on another box). A window will pop up warning you that changing the binary coefficient will affect the simulation results.

Click Yes to continue.

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Notice that the font colour changes from red to blue. Without closing the “Fluid Packages” click

on the icon to return to the simulation environment.

The same warning window will pop up again.

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Click Ok. A second window will pop up, asking you if you want to remain in “HOLDING” mode when entering the simulation environment. In HOLDING mode, the solver will not be active and computed values will not be updated.

Since we want new values to be computed, will click No. If you click Yes by mistake, click on

the green traffic light icon to activate the solver. Sometimes you may want to deactivate the solver (for example, if you want to change more than one parameter without the solver recalculating everything after each and every change); this can be done simply by clicking on the red traffic light icon.

The “Absorbed CO2” window should still be open. Click on Properties and scroll down to read off the “Std. Gas Flow” rate.

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The simulation prediction is getting closer to the experimental data! Return to the basis environment and try re-adjusting the binary coefficient. Your find your results should be these:

Binary Coefficient Std Gas Flow (N m3 / h)

69.676 1.00

69.000 1.97

68.000 5.52

68.500 3.29

68.600 2.97

68.550 3.13

68.560 3.09

68.558 3.10

Now that we have determined the binary coefficient for CO2 in Selexol it is time to repeat the process for CO and H2. Make sure you record your binary coefficients on a separate sheet (or notepad file in case, later on, you mistakingly change the value later on or reset the binary coefficients. Restore the PFD window and save your work.

Repeat the Process for H2

In the PFD window, left click on the mouse. While holding down the left mouse button, drag a box across the PFD model as shown below:

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Rather than respecifying the entire process from scratch, we can copy the present model and create a duplicate. Press Ctrl-C to copy the highlight model (or use File → Copy). Press Ctrl-V to paste a copy of the model onto the PFD. An outline of the new diagram will appear. Position the outline near the bottom of the window and left-click the mouse to place the copy.

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Our view is currently less than optimal, but we can position the units by clicking PFD →Auto Position All, as shown below.

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Your PFD should now look similar to the one below.

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Select the “CO2_IN-2” feed stream on the lower model and change the Stream Name to “H2_IN”. Click on Composition and change the mole fraction of “Hydrogen” to 1.0. The composition window will open up. Change the mole fraction of “CO2” to 0 and then click OK.

Close the window. Now, double click on the “Excess CO2-2” stream and change its name to “Excess H2”. Double click on the “Absorbed CO2-2” stream and change its name to “Absorbed H2”.

Now we must modify the component splitter because it is currently set to create a pure stream of CO2 (It would be great if UniSim could read our minds and carry out the correct modifications on its own…). Double click on the second component splitter “Splitter-2” to open up the component splitter window. Click on the Design tab, then select Splits. Change the split fractions of the “Absorbed_H2” stream to 1.0 “Hydrogen” and 0.0 “CO2”.

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Close the splitter window and open up the information window for the “Absorbed H2” stream. If you have carried out all of the changes properly, your numbers should match the ones shown below.

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The predicted solubility 0.0248 is not far off from the manufacturer’s data 0.03.

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As before, adjust the corresponding binary coefficient until your simulation’s prediction matches the experimental data. Record your number.

Save your work.

Repeat the Process for H2

Now you will repeat the procedure one more time to determine the binary coefficient for carbon monoxide in Selexol. If you need guidance, refer to the instructions in the step above.

Once you have completed this exercise. Save your work as 399tutor04.

Modifying the Binary Coefficients in the Full Model

Now that you have completed the tutorial model, open up the Integrated Design UniSim model (your main model) and change the binary coefficients so they match the numbers you obtained from the tutorial process model(s).

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You have now completed the first tutorial exercise!

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Change These Values…

Appendix

UniSim Customization

In Tools=>Preferences=>Resources=>Colours

Set “PFD Background” to white Set “PFD Label Text” to black Set “PFD Annotation” to black

In Tools=>Preferences=>Simulation

Uncheck “Confirm Before Adding if Active Correlations are Present” Uncheck “Enable Cross Hairs on PFD”

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